pcap files tcpdump produces by using the -r option with tcpdump or Wireshark. Output the tcpdump results to files by using the -w command. Use the same capture (destination IP) on this side of the router: $ tcpdump dst 192.168.2.100 Comparing what's sent on one side against what's received on the other side narrows down what traffic is filtered and therefore identifies the rules to edit. The capture on the server side displays what traffic is getting through-and you already know that the proper traffic is not arriving. Of course, the problem to resolve is that this is failing to happen. Ideally, the captures on each side of the router should be the same what's sent from one side should be passed through to the other side. Use tcpdump on the server side of the router to capture traffic addressed to the server. But how can tcpdump display whether the traffic is getting through the router and doing so on the correct interface? The capture displays where the traffic is trying to go. Remember, this capture occurs on the client segment. This can be compared against the existing filter rules as seen on the device and through a Nmap scan. The packet header displays the destination port and shows what IP address and port number the traffic needs to use. You can use tcpdump to intercept traffic originating from the client computer and destined for the remote server at 192.168.2.100. The following sections document both tools. Use either tcpdump or Wireshark to accomplish this. That's the data you're trying to collect. Recall, however, that a port number also gets exposed in the packet sniffer's output. The capture on the client side of the router displays packets addressed to the remote server's IP address. Intercept communicationsĪnother way to find out what destination ports the application expects to see is by capturing network traffic in each segment. The idea is to compare what ports the application wants to use against what ports it is allowed to use on the router. Next, find out what ports the application expects to see open. You'll get a list of ports for each interface on the router. Use the -oN option to cause Nmap to write the results to a file. This is a great opportunity to understand exactly how the router is configured. There may be more than just two interfaces in the router, so conduct any additional scans necessary to ensure the filters are not blocking (or permitting) the wrong traffic. Save the scan results, then compare the configuration of the filter against the scan results. Run the scan from each segment against the interface in the router connected to that segment. Begin by scanning both sides of the router to document the open ports. One logical troubleshooting step is to identify the available connections. You can use the following steps if you find yourself in the same situation. The general idea was to determine the ports the application expects to use and compare them against the ports the router permits through the interface. I decided to use Nmap, Wireshark, and tcpdump to help identify what changes needed to be made to the filter. I could have taken several approaches to re-establish connectivity. Many mistakes were made that allowed this failure to occur in the first place. This troubleshooting opportunity arose from a real situation I faced as an administrator. The analyst made multiple changes until they became frustrated and sent me an email saying, "I don't know what is wrong. The analyst did not back up the original filter configuration and kept no record of the changes. The analyst decided there was a problem with the router's filter and began making changes. I was the only administrator in the company, and as luck would have it, I'd taken the day off. The rules were ad hoc and not in a terribly logical order, and the relevant application communicated between the client and server using non-standard ports. A router with strict packet filtering rules sat between the two segments. In my role, I inherited this environment and was told to accept the fact that non-administrators had administrative privileges.Ī client device sat on one internal segment, and a database server resided on another. Unfortunately, this individual did not have a solid understanding of these devices or their services, such as firewalls and packet filters, logs, or routing. In this scenario, a business analyst had administrative privileges on all network devices (routers, switches, servers, and clients).
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